In the United States alone, over 1.3 million miles of buried steel main-line pipe are used for the transport of natural gas alone, many at high pressures on the order of 1000 psi. Trunk lines for natural gas products add to this total. There are also about 170,000 miles of pipeline for transport of crude oil and refined products. There are a significant number of pipeline failures reported each year with a few involving loss of life or significant property loss.
Corrosion of the pipe material is one of the main causes of pipeline failure. Corrosion is an electrochemical process involving metal oxidation of the pipe and mass and charge transport with an electrode via a surrounding electrolyte. The charge transport implies that an electrical current flows between locations on the pipe and from the pipe to one or more external electrodes. A metallic pipeline can become an electrode and the soil act as an electrolyte so that the pipeline buried in soil provides the elements of an electrolytic cell.
Some corrosion arises from the naturally occurring processes at specific locations on the pipe involving electrical current flow into the ambient soil electrolyte via the corrosion reaction. Corrosion can also be accelerated by voltages applied to a local region of the pipe by man-made structures, including local transit systems, power distribution systems and other terrestrial sources of stray voltages and currents.
Early detection and control of corrosion are necessary to maintain the integrity of a pipeline and reduce the likelihood of a pipeline failure. In order to reduce the likelihood of a pipeline failure, U.S. Federal Law requires that pipelines are periodically tested for indications of corrosion activity.
Electrical potential measurements are commonly used to assess the efficacy of corrosion prevention strategies. Buried oil and gas transmission pipelines are protected from corrosion by a combination of cathodic protection and coatings that reduce the current requirements for cathodic protection. Cathodic protection (CP) is a method of preventing metal corrosion by suppressing the electrochemical corrosion reaction. The metal being protected is forced to be a cathode by either impressing a small current upon it, or by placing it in electrical contact with a sacrificial anode, i.e. a metal that is more easily oxidized than the protected metal. These methods of CP are referred to as Impressed Current Cathodic Protection (ICCP) and Sacrificial Anode Cathodic Protection (SACP), respectively.
One of the advantages of CP is that it can provide protection without changing the immediate physical environment of the structure. By its nature, CP provides the correct electrochemical conditions to control the corrosion process without requiring full access to the material to be protected, thus preserving the visual appearance and structural integrity of the structure.
Regular inspections, such as described in the NACE (National Association of Corrosion Engineers) International Recommended Practice for External Corrosion Direct Assessment (ECDA), are required to ensure the integrity of the pipelines. The methodology behind ECDA relies heavily on close-interval surveys of on-potentials (where the CP system is connected) and off-potentials (where the CP system is disconnected).
The procedure used for a close-interval survey is to place a reference electrode in contact with the soil surface above the pipe and measure the electrical potential difference with respect to a connection to the pipe. Since it is impractical to connect to the pipe at each point where a measurement is taken, a length of wire is used to reach convenient connection points. The measurement location and electrical potential are recorded and the next measurement is taken. In some cases, the distance between measurements is the length of one pace of the person obtaining the data. In other cases, an A-frame is used to control more precisely the distance between potential readings.
One available system superimposes a single-frequency signal (937.5 Hz with a maximum output of 750 mA or 4 Hz with a maximum output of 3 A) on the current supplied for cathodic protection. A magnetometer, tuned to the imposed frequency, is used to measure the current flowing in the pipe. A discontinuity in the current is associated with coating flaws. Although such instruments can detect certain coating flaws, such systems can be insensitive to large coating flaws. Another available system applies an AC signal, containing many frequencies, to the pipe, and the corresponding current is detected at various locations using a magnetometer. The ratio of input potential and output current yields a frequency-dependent impedance. The magnetometers can be used as well to map the current distribution associated with application of cathodic protection.
However, magnetically-assisted impedance-based systems have not gained acceptance in the field. One factor inhibiting acceptance of the magnetically assisted impedance technique within the industry is that it requires depolarization of the pipeline because the magnetically-assisted impedance-based technique cannot be employed while the pipe is under cathodic protection. Even after the impressed current is removed from the pipe, depolarization may take hours. Additionally, interpretation of the impedance results requires a detailed electrical circuit model, which is unlikely to be correct, and requires low-frequency data, which takes a long time to collect. For example, a measurement of three 0.01 Hz (10 mHz) cycles requires 300 seconds. Accordingly, methods which provide measurement under normal operating conditions and provide more sensitivity for above ground assessment of the condition of buried pipe are desired.